1. Field of the Invention
The present invention relates to the field of optical scientific instrumentation. More specifically, the present invention relates to a dual tilt control system that includes a soft flexure beamsplitter assembly as incorporated in a Fourier-Transform infrared (FTIR) scanning interferometer.
2. Discussion of the Related Art
An optical interferometer used in a scientific analytical instrument relies on the interference of superimposed optical beams as part of the interrogation means. When configured as a Michelson Fourier-Transformed infrared (FTIR) instrument, the optical output of the interferometer is called an interferogram. The FTIR interferometer itself often includes a beamsplitter and two mirrors, one that is conventionally stationary, and one which is conventionally mobile. The mobile mirror moves along the optic axis while staying optically perpendicular to the light beam at all times. The movement of the mobile mirror is often desired to be feedback controlled in order to hold the mirror velocity constant so that the analytical radiation that passes through the interferometer produces an accurate interferogram. Conventional interferometers have a moving mirror assembly that includes a linear ball bearing, air bearing, slide bearing, or a flexure bearing and is often driven by a linear motor (e.g., a coil coupled to a permanent magnet) to provide velocity control.
Motion and a resultant velocity of the mobile mirror in a conventional system can be tracked by a positioning monochromatic beam of optical radiation operating in conjunction with the analytical radiation beam passing through the interferometer optics. The configured monochromatic beam (e.g., laser) is thus also often partially reflected and partially transmitted through the configured beamsplitter of the interferometer, and because of the design, the split beams are reflected from the conventionally fixed mirror and the conventionally mobile mirror and recombined at the beamsplitter.
The recombined beams at the beamsplitter are thereafter directed to a detection means that can thereby determine the tilt, position, and/or velocity of the mobile mirror along its longitudinal translation axis. With respect to tilt, the deviations in the phases of the components of the recombined beams can be used to indicate a misalignment of the mobile mirror with respect to a perpendicular of the designed longitudinal axis for the optical element. If such deviations are deleterious, a tilt servo controller can apply corrective forces to the support of the mobile mirror so as to realign the face of the mirror.
However, it is to be appreciated that because the angle of the beamsplitter is often designed to bisect the angle of configured mirrors that help modulate the source beam, if either of the mirrors or the beamsplitter tilts away from the correct angle, the modulated light signal generated by the interferometer can be quickly degraded. Tilt errors that are more than about 50 arc seconds not only reduce the quality of the modulated light signals but tilt errors larger than 50 arc seconds also causes the modulated light signals to reduce in magnitude to the point of effectively disappearing, causing the interferometer control system to stop operating.
Thus, conventional systems are often configured to be precisely aligned with mechanical adjustments that must stay correctly adjusted even if the system is shipped around the world. This has resulted in expensive stiff precision mechanical interferometer systems that sometimes need adjustments in the field after shipping shocks has shifted the alignment of the critical flat optical surfaces.
In operation, many of such conventional systems use active control systems (i.e., dynamic alignment) to control mirror and beamsplitter tilt as the interferometer scans and collects a desired spectral data. Such systems can only operate if the interferometer is scanning under the control of a laser based velocity control servo that all typical scanning interferometers use. Thus, the static alignment of the interferometer is called upon to be good enough at less than about 50 arc seconds to produce useable optical feedback signals before any active control system(s) can be utilized to control interferometer scan speed, tilt error and then data collection.
Background information on a similar interferometer system that utilizes conventional laser based control servos, is described and claimed in, U.S. Pat. No. 5,883,712, entitled, “INTERFEROMETER OF AN INFRARED SPECTROMETER WITH DYNAMIC MOVING MIRROR ALIGNMENT” issued Mar. 16, 1999, to John M. Coffin, including the following, “[i]n accordance with the present invention, an interferometer for an infrared spectrometer provides dynamic alignment of the moving mirror to maintain precise alignment between the moving mirror and the fixed mirror. The alignment of the moving mirror in this manner maximizes the stability of the interferometer while achieving high levels of output accuracy despite vibrations due to the movement of the moving mirror on its bearings and vibrations transmitted from external sources to the interferometer. The dynamics of the mounting of the moving mirror allow the position of the mirror to be controlled with high accuracy even in the presence of relatively high frequency vibrations. The structure of the interferometer and of the detectors and controls for maintaining the alignment of the moving mirror are nonetheless simple in construction and contribute relatively little additional bulk or weight to the interferometer.”
Accordingly, the present invention addresses the need for an inexpensive beamsplitter mounting assembly that simultaneously preserves the optical flatness of the coupled optical components configured in an interferometer instrument. To provide for time efficient and reliable data when incorporating such soft mounting configurations, the present invention is also directed to an improved tilt control system that includes combining a wide range dynamic tilt control system with a phase measurement tilt control system so as to maintain beamsplitter alignment over a large range of tilt errors that, as a system, is more rugged than typical interferometer instruments.